1. Technical Field
The invention relates to the field of devices, in particular guidewires and catheters, needed for minimally invasive interventions.
2. Related Art
Minimally invasive interventions in the human body require guidewires and catheters. These are available in a multitude of forms, sizes, configurations and mechanical characteristics for procedures guided by X-ray imaging.
These catheters and guidewires are not applicable to magnetic resonance tomography (MRT)—guided procedures as they usually contain metals. This leads to artefacts which makes evaluation of the images difficult or even impossible. Furthermore, this conceals the risk of inductive heating in the magnetic field leading to a potential risk to the patient.
Three essential requirements have to be fulfilled by devices to make them suitable for use in MRT:                1. They must not contain any long metal parts, e.g. metal fabric for catheter reinforcement or wire cores for guidewires.        2. They need to be visible over optimally their full length in the MRT image so that the position of the device is clear in relation to the organ(s).        3. As the local resolution in real-time MRT currently does not allow a direct imaging of catheters and guidewires, clearly visible effects have to be generated along the length of the device.        
Such effects on the one hand should be strong enough to render the device well visible in the MRT image but on the other hand weak enough not to make important structures in the vicinity unidentifiable.
The strong magnetic field of a magnetic resonance tomograph makes the absence of any ferromagnetism a precondition for the use of devices within this equipment. This excludes e.g. many standard catheters which can be attracted and misguided by the strong magnetic field. Materials solely composed of polymers fulfil the prerequisite of absence of ferromagnetism.
The minimal size of devices used for intervention such as guidewires or catheters causes a special problem for application in magnetic resonance tomography and especially for rapid imaging. The faster imaging is done in MRT the lower is the local resolution. In order to make visible such a small item as a catheter which shows up as a dark object due to the relative lack of hydrogen protons, appropriate high-resolution and thus slow imaging is necessary. Furthermore, it is very difficult to make visible such a small item in the usually prepared layer thickness of around 10 mm so that on the one hand it is positioned within the recorded layer and on the other hand it does not become invisible due to partial volume effects.
A solution to this problem resides in markers which lead to a local extinction of the signals in the MRT image and therefore allow easier identification and visualization of the device. For this purpose generally materials which possess a susceptibility (magnetizability) different from water are suitable. The markers have to be applied locally in order to avoid dependence of the marker on the orientation of a device in the main magnetic field. Rare earths which are used as MRT contrast agents have been used for this purpose in higher concentrations. They have been applied or introduced, both locally as markers as well as for catheter fillings to obtain better visibility of devices in the MRT.
A major disadvantage of these substances used so far as local markers resides in their rather high mass required to achieve a sufficient marking effect in the MRT. This is reflected e.g. in the fact that much more complicated techniques for conducting current along a wire within the device, or which mount micro-inductors on the catheter, have been developed although resultant safety issues such as heating from the radio frequency field have not yet been resolved at all. These heating effects occur when electric conductors such as metals are exposed to the radio frequency field over a longer distance in the MR tomograph. If resonance occurs, a summation of the irradiated radio energy over a standing wave results, with the possible consequence of a substantial heating of the conductor.
Known single-stranded homogenous non-metal materials exhibit major disadvantages of their material characteristics in comparison to common metal cores regarding stability, flexibility and elasticity, e.g., materials with high stability mostly possess low flexibility and/or elasticity. Consequently it has not been possible to date to replace the common metal core by an MRT compatible and visible material.